a. Field of the Invention
This invention relates in general to hygrometer apparatus which includes electrical circuitry to measure moisture condensed as dew or frost on a chilled mirror which is cooled by a cryogenic source, and to the method of using the same. More particularly, the invention concerns an apparatus and method for very fast and accurate measurement of moisture in a gas over a wide range of dew or frost point temperatures and gas pressures.
b. Description of the Prior Art
Hygrometry, the measurement of moisture in gases, has always been important to industrial processes and atmospheric research, but recently the importance of such measurements has increased. As microelectronic feature sizes move into the submicrometer realm and super critical fluid extraction based on high-purity carbon dioxide becomes the accepted method for environmental and pharmaceutical extraction, the low parts-per-billion (ppb) moisture range is becoming the maximum acceptable level for moisture and other contaminants in specialty gases.
The aerospace industry has similar moisture contaminant standards for space station systems. Atmospheric moisture measurements are important to understanding the role moisture plays in the chemistry of global warming and ozone depletion.
There has been a continuing effort to increase the sensitivity, accuracy and response time of moisture analyzers, and especially performance of the dew point or frost point chilled mirror hygrometers, to meet these challenges.
As is explained in Bisberg U.S. Pat. No. 3,623,356, dew point hygrometers are frequently used to determine the moisture content of a gas sample. Typically a mirror is exposed to the gas sample and cooled to the dew or frost point temperature of the gas. Formation of dew or frost on the mirror is detected by means of a light source and a light sensitive detector responsive to light reflected from the mirror. The detector develops an error signal which is used to control the current to a thermoelectric cooler attached to the mirror, to maintain a predetermined thickness of condensate on the mirror surface. A temperature measuring device attached to the mirror is employed for measuring the temperature of the mirror, thus indicating the dew or frost point of the gas sample. In this reference, the mirror, optics, and electrical circuit constitutes a thermo-optical servo system which functions to maintain a constant reflectance at the mirror surface, hence a condensate equilibrium at the dew or frost point temperature. Its operation is based upon the principle that an equilibrium exists between the water vapor pressure in the atmosphere and a water/ice surface at a unique temperature, the dew/frost-point temperature.
Most hygrometers employ two light sources and two photodetectors as coordinated pairs to reduce the effects of temperature on source brightness and/or detector efficiency. One pair provides an output which is proportional to the light which is specularly reflected from the mirror. The other pair, provides a reference output which is used to correct for temperature-caused changes.
The most common prior art method for cooling the mirror is thermoelectric, by means of a Peltier junction, which lowers the surface temperature as a function of applied voltage. Peltier junctions, however, become rapidly less efficient at lower temperatures, and achieving very low frost points requires massive multistage systems which consume exponentially larger amounts of power as the frost point decreases. With the Peltier junction, it becomes impractical to achieve frost point temperatures much below about -80.degree. C. Further, as frost point temperature becomes lower, the response time of thermoelectrically cooled hygrometer becomes very slow.
Industrial chilled mirror hygrometers commonly utilize thermoelectric cooled mirrors. However, their limited measurement range and slow response makes them less desirable for real-time on line monitoring of moisture levels, especially in the critical area of semiconductor manufacturing.
Cryogenic cooling of the hygrometer mirror as compared to thermoelectric cooling, allows operation to much lower moisture levels, with much faster response time. Cryogenically chilled mirror hygrometers have been developed by Cambridge Systems Inc, the Naval Research Laboratory (NRL), and the National Center for Atmospheric Research (NCAR), and others. The cryogenic hygrometers use a cryogen, such as liquid nitrogen or freely boiling Freon 13 chlorofluorocarbon coolant, as the heat sink. In these instruments, generally, a mirror is thermally connected to a cryogenic heat sink by means of a thermo-conductive rod. An electrically resistive coil wound around the thermo-conductive rod provides heating to raise the temperature of the mirror to the dew or frost point. In all of these prior art cryogenic instruments, the thermo-conductive rod is attached to the mirror at one end, and is immersed directly into the cryogen at the other end of the rod. To maintain performance at all levels of cryogen, the rod is inserted horizontally into the cryogen container, most typically a dewar, near the bottom of the container. This requires a container with an orifice created in its body, a complex and costly procedure since the dewar cryogen container typically comprises two nested and spaced apart shells, with the space between them being evacuated to obtain the maximum thermal insulation. Thus, in making a penetrated dewar, the vacuum seal must be broken, an opening formed through both shells, the space between the shells evacuated and resealed. A further complication in the resulting system is the loss of cryogen efficiency due to thermal losses caused by the stem penetration through the dewar. Furthermore, the prior art fixed thermal transfer arrangement, in which a mirror is thermally connected to a cryogenic heat sink by means of a thermo-conductive rod, allows only a limited temperature measurement range over which the instrument can operate in an optimum manner.
In the above noted prior art cryogenic instruments, control of condensation on the mirror is accomplished by a control circuit in which the input voltage corresponds to the moisture condensate level, and in which the output voltage to the heating coil controls the mirror temperature. This is known as a Type Zero proportional control circuit which responds to condensation level (proportional) and the rate of change of condensation (lead). Because it is relatively simple, and because the physical parameters involved vary over the operation range, such a Type Zero proportional control circuit is able to provide effective control over only a limited moisture measurement range. Outside of that range, it becomes slow or subject to oscillation.
Because of these thermal and circuit limitations, the prior art cryogenic instruments have been able to operate only at relatively low frost points, typically below -10.degree. C. For airborne research at high altitude, this is acceptable. However, these prior art instruments are not capable of functioning accurately and efficiently in the wider temperature ranges needed to meet modern industrial demands.